V. Focused Fundamental Research - EERE - U.S. Department of ...

V.C.1 Nanoscale Composite Hetero-structures: Novel High Capacity Reversible Anodes (U. Pitts)
Kumta – U. Pitts
Figure V - 76: Charge capacities of Si/C based composite using PVDF, and
the two novel polymer binders.
Synthesis of amorphous Si films directly on copper
foil by electrochemical reduction of silicon salts.
Formation of amorphous Si film on copper foil, obtained
by electrochemical reduction of silicon salts, has been
confirmed from Raman spectroscopy and scanning
electron microscopy (SEM). A broad peak at ~ 485 cm -1
was observed in the raman spectrum which is
characteristic of a-Si. No sharp peak at 520 cm -1
corresponding to crystalline silicon was observed
indicating that the deposited films were mostly amorphous.
The amorphous silicon (a-Si) films were electrochemically
charged and discharged at ~400 mA/g current to evaluate
their potential as suitable anodes for Li-ion batteries. As
shown in Figure V - 77, a first discharge capacity of ~3400
mAh/g was obtained with an ICL of 60% probably due to
impurities arising from the chemical reduction of the
supporting electrolyte used and the expected surface
oxidation of the amorphous Si. Efforts are in place to
reduce it to the desired 15% level.. However, after the 1st
cycle, a stable reversible capacity of ~1300 mAh/g was
obtained. The columbic efficiency varied from 94% to
98% from 2nd to 5th cycle, after which it improved and
remained close to the desired goal of 99.9% for the
remaining cycles. A capacity fade of ~0.016% per cycle
was observed resulting in a capacity of ~1260 mA/g at the
end of the 100th cycle. This approach of developing thin a-
Si films directly on Cu eliminates the use of binders and
conducting agents, rendering the process simple, facile,
and amenable to large scale manufacturing.
Figure V - 77: Cycling data for the deposited amorphous film cycled at ~400
mA/g.
Conclusions and Future Directions
The nc-Si/C composites synthesized by cost effective
processing techniques such as HEMM, CVD, chemical
reduction or electrochemical reduction processes exhibit a
high reversible capacity of ~800-2000 mAh/g. However,
the Si/C nanocomposite synthesized by these techniques
show high ICL, low coulombic efficiency or limited
structural stability depending on the synthesis procedure.
In order to improve the coulombic efficiency, rate
capability, or long term structural stability, various
coatings, conducting additives and electronically
conducting dopants have been pursued. A unique binder
free hybrid Si/VACNTs based nanostructured electrode on
INCONEL 600 alloy was synthesized using a simple cost
effective CVD approach. The Si/VACNT exhibited a low
irreversible loss (~16%), a reversible capacity ~1500
mAh/g and excellent rate capability. Addition of
conducting dopants and additive layers with Si/C
nanocomposites improved the coulombic efficiency and
reduced the irreversible loss. Amorphous Si films obtained
by electrochemical reduction of silicon salts shows a
reversible capacity ~1300 mAh/g with excellent stability
due to the improved adhesion of the deposited Si with the
underlying copper foil. High strength thermoplastic and
elastomeric binders were developed to improve the
capacity retention by minimizing the colossal damage
occurring due to the large volume changes upon lithium
alloying and de-alloying.
Future work will be dedicated to improve the
structural stability and couloumbic efficiency of nc-
Si/C/CA or Si/CNT hybrid structures using core-shell
morphology and developing new elastomeric binders. In
addition, scale up activities using Si/C/CA composite and
Si/CNT hybrid structures will be initiated and performed.
Additionally, chemical reduction and mechano-chemical
reduction approaches will be investigated to generate
Energy Storage R &D 538 FY 2011 Annual Progress Report